1. Field of the Invention
The present invention relates to a current to voltage converter, and particularly to a current to voltage converter for converting a photocurrent into a voltage to output.
2. Description of the Related Art
A photocoupler is used to electrically separate an input signal from an output signal. A photocoupler has a light emitting device and a light receiving device. A light emitting device converts an electric signal input into an optical signal and a light receiving device converts the optical signal into an electric signal to output. For example, as for a photocoupler that transmits a digital value of “H” or “L” level according to an existence of an optical signal, a current to voltage converter (I-V converter) for converting a photocurrent generated in a light receiving device including a photodiode into a voltage is used. Such I-V converter is disclosed in Japanese Unexamined Patent Application Publication No. 2003-198476 and Japanese Unexamined Patent Application Publication No. 07-193474.
FIG. 5 shows the I-V converter disclosed in Japanese Unexamined Patent Application Publication No. 2003-198476. In the I-V converter of the related art, a voltage Va based on a photocurrent generated by a light receiving device is generated by a voltage generator 20. In a threshold voltage generator 70, a threshold voltage Vth is generated for evaluating whether the voltage Va generated by the voltage generator 20 is “H” or “L” level. Then a comparing unit 60 compares the Va and Vth to output a digital signal of “H” or “L” level to a subsequent circuit.
In the I-V converter, the threshold voltage Vth for evaluating “H” and “L” is generated as follows. If a constant current source I4 shown in FIG. 5 is in on state, the threshold Vth is set to Vth=Va−(I4−I3)*R4. If I4 is in off state, the threshold Vth is set to Vth=Va+I3*R4. With no photocurrent input, the constant current source I4 is referred to as off state and the threshold voltage Vth is set higher than the voltage Va. On the other hand, with photocurrent input, the constant current source I4 is referred to as on state and the threshold voltage Vth is set lower than the voltage Va (see FIG. 6). By changing the threshold in this way, a pulse distortion at a rising and a falling edge of an input signal is prevented in Japanese Unexamined Patent Application Publication No. 2003-198476.
However in the I-V converter, both Va and Vth are unstable at a power-on. Therefore as shown in FIG. 7, when evaluating to be Va>Vth even with no photocurrent input and a signal of a state with a photocurrent input is output, also as shown in FIG. 8, it has now been discovered that a signal of a state with no photocurrent input is sometimes output even if there is a photocurrent input.